drm/i915: Do an unlocked wait before set-cache-level ioctl

Since a change in cache level is likely to trigger an unbind, avoid
waiting under the mutex by preemptively doing an unlocked wait.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170119082211.21257-1-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>